1. Field of the Invention
The present invention relates to an exhaust gas purifying catalyst to be used for an exhaust system of a lean burn gasoline engine, or the like, specifically, it relates to a catalyst having an Ir powder.
The present application is based on Japanese Patent Application No. 2000-106125, which is incorporated herein by reference.
2. Description of the Related Art
Conventionally, as an Ir powder in this kind of a catalyst, those having a particle size distribution with a peak in a particle size Dxe2x89xa725 nm have been used.
However, the conventional catalysts involve a problem in that the oxidization performance with respect to a CO, and an HC is low in the case of driving the engine by the theoretical air fuel ratio. The low oxidization performance is considered to be derived from a low dispersion degree because the Ir powder with the particle size distribution contains many particles with a relatively small specific surface area, and thus with a relatively large particle size in view of improvement of the oxidization performance.
Accordingly, an object of the invention is to provide a catalyst, capable of providing a good oxidation performance in driving an engine by the theoretical air fuel ratio, and the excellent durability even in the case it is directly exposed to an oxygen excessive atmosphere under a high temperature.
In order to achieve the object, according to the invention, an exhaust gas purifying catalyst having an Ir powder, wherein the Ir powder has a two peak particle size distribution, with a peak in a particle size D greater than 25 nm region, and the other peak in a particle size D less than 25 nm region, respectively, is provided.
In the Ir powder particle size distribution, existence of a peak in a particle size D less than 25 nm region means existence of many Ir particles with a large specific surface area, that is, a small particle size. The many Ir particles with a small particle size have a large dispersion degree, and thus the catalyst provides a good oxidation performance in a drive of an engine by the theoretical air fuel ratio so as to maintain the CO, HC purifying ratio at a high level.
In contrast, existence of a peak in a particle size D greater than 25 nm region means existence of many Ir particles with a small specific surface area, that is, a large particle size. The many Ir particles with a large particle size can hardly be oxidized even in the case they are directly exposed to an oxygen excessive atmosphere under a high temperature so as to provide the excellent durability, and thereby, the catalyst also maintains a high NOx purifying ratio after aging.